The interactions between the tomato plant (Lycopersicon esculentum) and its pathogen, Pseudomonas syringae pv. tomato, provide an excellent system for the study of plant gene expression during host-pathogen interactions. Genes which are expressed in a disease-resistant plant ("defense" genes) during bacterial pathogen infection will be isolated, and their structure and gene expression programs will be determined. The sequences which are important in the regulation of "defense" gene expression will be identified by deletion mutant and gene fusion analysis. Intact and modified genes will be introduced into the disease-sensitive tomato genome; transformed plants will be regenerated and analyzed for the structure, organization, and expression of the "defense" gene. In addition, transformed plants will be tested for the expression of the disease-resistance phenotype. These experiments will provide new and basic information about how plant gene expression is regulated during a biological stress in a higher eukaryotic.